The X-ray structure of photosystem II reveals a novel electron transport pathway between P680, cytochrome b 559 and the energy-quenching cation, ChlZ +

Abstract

When water oxidation by photosystem II (PSII) is impaired, an oxidized chlorophyll (ChlZ+) is formed that quenches excitation and may prevent photodamage. Both the identification of this Chl+ and the mechanism of its oxidation and reduction are controversial. Using the available X-ray structures of PSII we calculated the efficiency of two proposed quenchers, ChlZ+(D1) and ChlZ+(D2). Of these two, only ChlZ+(D1) can quench to the degree observed experimentally. We also identify a chain of closely spaced pigments in the structure from Thermosynechococcus vulcanus that we propose to form a novel electron transport pathway between ChlZ(D1), β-carotene, P680+ and cytochrome b559